1. Field of the Invention
The present invention relates to contact lenses, and more particularly to a bifocal lens of having a soft plastic material in contact with the eye.
2. Description of the Prior Art
Persons suffering from presbyopia require bifocal type lenses to enable them to focus on both distant and near objects. Such lenses require areas having two different powers and correction. Bifocal lenses are known for both conventional glasses, and for contact lenses that are inserted directly to the eye. The most common bifocal contact lens is of the alternating vision type in which the distance power portion is ground to permit the target to be viewed when looking straight ahead. For closeup viewing, the lens must move over the surface of the eye to place the closeup power portion directly over the pupil, with the distance power portion then out of the line of sight.
It is therefore common to form the lens from a rigid plastic material, such that the lower edge of the lens contacts the lower eyelid when the eye is rotated downward, causing the lens to move upward. Gravity and the upper eyelid returns the lens to the distance position when the eye is rotated to the straight ahead position. It has been found that many persons have difficulty with a lack of comfort of hard plastic lenses, and prefer those of soft materials. Heretofor, soft plastic bifocal lenses have not been successful. The problem is that the edges of a lens tend to move under the eyelid, rather than being moved.
Typical prior art patents of bifocal contact lenses include U.S. Pat. Nos. 4,874,234; 4,618,229; 3,597,055; 3,560,598, and 3,440,306, and Japan patent No. 63-95,415.
There is a need for soft bifocal lens that will be moved when a user's eyes shift from a far vision position to a close vision position, and that will be comfortable for the user.